The present invention is a method for stabilizing a mixture comprising polydiorganosiloxane and an alkali metal. The method comprises contacting a silyl phosphonate described by formula R.sup.1.sub.a (OH).sub.b (OSiR.sup.2.sub.3).sub.3-a-b P.dbd.O, where each R.sup.1 and R.sup.2 is an independently selected hydrocarbyl radical comprising less than about twelve carbon atoms, a=1 or 2, b=0 or 1, and a+b=1 or 2, with a mixture comprising a polydiorganosiloxane and an alkali metal.
Polydiorganosiloxanes are used in the form of fluids and are crosslinked to form, for example, silicone rubbers and resins. Many of the uses of polydiorganosiloxanes fluids, silicone rubbers and resins require that the material be stable when exposed to high temperatures. Typically polydiorganosiloxanes are prepared by the process of heating low molecular weight linear polydiorganosiloxanes and cyclic polydiorganosiloxanes at a temperature above about 100.degree. C. in the presence of a strong base such as potassium hydroxide or potassium silanolate to form an equilibrium mixture. Other known alkali metal catalyst for this type of polymerization include sodium hydroxide, cesium hydroxide, lithium hydroxide, and their corresponding silanolates or siloxanates. In the case of cyclic polydiorganosiloxane polymerization, a ring opening reaction occurs with the formation of linear polymers. Typically the product of this polymerization reaction is an equilibrium mixture comprising a linear polydiorganosiloxane as a major portion and cyclic polydiorganosiloxane as a minor portion. The presence of the cyclic polydiorganosiloxane in products is undesirable because it can have sufficiently high vapor pressure to cause problems during use, and therefore these cyclics are typically removed. The most convenient method of removing these cyclic siloxanes is by heating under reduced pressure. However if the basic caualyst's activity is not hindered additional cyclic siloxanes will be generated during the distillation process in an attempt to maintain equilibrium conditions. Therefore, it is important in the preparation of linear polydiorganosiloxanes to stabilize the basic catalyst to both improve yield of the process and to stabilize final products.
Various methods of stabilizing the basic catalysts which may be present in polydiorganosiloxanes have been used in the past. For example, strong acids such as hydrochloric acid and sulfuric acid have been suggested as useful. But since strong acids are also known equilibrium catalyst for cyclic and linear polydiorganosiloxanes, the amount of acid must be controlled carefully to prevent the presence of excess which can cause degradation of the polydiorganosiloxanes.
Triprotic acids such as phosphoric acid and arsenic acid have been proposed as useful for stabilizing basic catalysts comprising alkali metal compounds, since they can serve as a buffering agent. The use of arsenic acid is generally not desirable due to its toxicity. Although phosphoric acid has good buffering capability and low toxicity, its use as a neutralizing agent is limited due to its insolubility in polydiorganosiloxanes. To be an effective catalyst stabilizer the stabilizing material needs to be soluble in the polydiorganosiloxanes so that it can contact the alkali metal which is often located in the equilibrium mixture on the terminal silicon atoms of the polydiorganosiloxane.
Razzano et al., U.S. Pat. No. 4,177,200, describe a process for preparing silyl phosphates which are soluble in polydiorganosiloxanes. The silyl phosphates are prepared by the reaction of a linear siloxane and a phosphorous oxyhalogen or phosphoric acid. Razzano et al. teach the silyl phosphates are useful for the continuous neutralization of alkali metal hydroxides in an equilibrium mixture of polydiorganosiloxanes.
Petersen, U.S. Pat. No. 4,177,220, describes the use of a catalyst suitable for use in a process for preparing silyl phosphates. The process taught by Petersen comprises reacting phosphoric acid with a linear polysiloxane in the presence of a silyl phosphate. The presence of the silyl phosphate allows the reaction to initiate quickly and smoothly. A portion of the silyl phosphate from a previous run can be used as the catalytic amount of silyl phosphate. Petersen teaches the silyl phosphates prepared by the described process can be used as a neutralizing agent for neutralizing alkali metal hydroxides and bases in siloxane solutions.
Beck et al., U.S. Pat. No. 5,041,586, teach the preparation of silyl phosphate mixtures by adding phosphoric acid to hexamethyldisiloxane under reflux. The resulting mixture is report to comprise 10 to 30 weight percent of monosilyl phosphate, 65 to 85 weight percent of disilyl phosphate, and 2 to 7 weight percent of trisilyl phosphate. This mixture is reported to be useful in stabilizing basic polymerization catalyst such as potassium hydroxide or potassium silanolate, when such catalyst are present in polydiorganosiloxane mixtures.
Beck et al., U.S. Pat. No. 5,099,051, teach the preparation of siloxanyl-phosphate mixtures by reacting a cyclopolydimethylsiloxane and a silylphosphate mixture, as described in Beck et al., U.S. Pat. No. 5,041,586. The siloxanyl-phosphate mixtures are reported to be useful in stabilizing basic polymerization catalyst such as potassium hydroxide or potassium silanolate, when such catalyst are present in polydiorganosiloxane mixtures.
Herberg et al., U.S. Pat. No. 4,551,515, teach that agents such as phosphoric acid, tris(chloroethyl)phosphite and silyl phosphate can be use to neutralize basic polymerization catalyst present in compositions comprising polydiorganosiloxanes.
The present inventors have discovered that silyl phosphonates can be use to stabilize mixtures comprising polydiorganosiloxane and alkali metal. These silyl phosphonates are readily soluble in the polydiorganosiloxanes. Furthermore, and quite unexpectly the silyl phosphonates have a greatly improved shelf life diluted in cyclic polydiorganosiloxanes, in comparison to silyl phosphates diluted in cyclic polydiorganosiloxanes. This improved shelf life allows for a masterbatch dilution of the silyl phosphates to be made and used for an extended time in manufacturing processes. Another advantage of the present method is that in comparison to silyl phosphates which may contain residual phosphoric acid that can potentially form silicone gels, this potential does not exist with the silyl phosphonates.